Merge pull request #136 from geoffharcourt/geoffharcourt-issue_135

Only define Impressionable::ClassMethods::DEFAULT_CACHE when undefined

app/models/impressionist/impressionable.rb

@@ -5,7 +5,11 @@ module Impressionist
     module ClassMethods
       attr_accessor :impressionist_cache_options
 
-      DEFAULT_CACHE = { :counter_cache => false, :column_name => :impressions_count, :unique => false }
+      DEFAULT_CACHE ||= {
+        :counter_cache => false,
+        :column_name => :impressions_count,
+        :unique => false
+      }
 
       def impressionist_counter_cache_options
         @impressionist_cache_options ||= {}